J Bagh College Dentistry Vol. 27(3), September 2015 The effect of different
Restorative Dentistry 1
The Effects of Different Investment Materials on
Dimensional Accuracy and Surface Roughness of
Thermosens Maxillary Complete Dentures
Thekra I. Hamad B.D.S, M.Sc, Ph.D (1)
Abdalbasit A.Fatihallah B.D.S, M.Sc, Ph.D (1)
Ali J.Abdulsahib B.D.S, D.D.S, M.Sc (2)
ABSTRACT
Background: Limited data are available on the dimensional stability and surface roughness of ThermoSens, which is a
material used in denture processing. This study aimed to measure the vertical teeth changes and surface roughness
of ThermoSens dentures prepared using three different investment materials.
Materials and methods: For the dimensional changes test, 30 complete maxillary dentures were prepared using
different investment methods: group I, dental stone; group II, silicone putty; and group III, a mixture of dental stone and
plaster (ratio, 1:1; n = 10 for each group). Eight screws where inserted, four for each side of the denture: two were
attached to the buccal surface of the canine and first molar, and the other two were attached in the flange areas of
the canine and first molar in line with the previously mentioned screws. Measurements were made using a micrometer
microscope in the wax stage before flasking and in the deflasking stage. The above investment techniques were also
used to prepare samples for a surface roughness test (n = 10 per group). These samples were prepared according to
the specifications of the American Dental Association. Data were examined using analysis of variance (ANOVA) and
the least significant difference (LSD) test.
Results: One-way ANOVA and LSD revealed that dimensional changes significantly differed among all groups, except
that the vertical teeth changes on the left side did not differ between groups I and II for both the canine and molar
regions. Surface roughness was significantly higher in group I than in group II, and in group III than in group II.
Conclusion: The use of putty silicone for investing ThermoSens complete dentures reduced dimensional changes and
resulted in dentures with a better fit. Surface roughness could be reduced by the addition of a putty silicone layer over
the denture before the addition of the second investment layer during denture processing.
Keywords: Investment material, dimensional changes, surface roughness, ThermoSens. (J Bagh Coll Dentistry 2015;
27(3):1-7).
INTRODUCTION
Denture bases dimensional stability during
processing and in service is important for dentures
to accurately fit the underlying foundations and
satisfaction of the patient. In general, if the denture
is properly processed, the original fit and
dimensional stability are good, regardless of the
denture base material used.(1) Currently,
poly(methyl methacrylate) (PMMA) is widely
used in prosthetic dentistry for the construction of
complete and partial dentures.(2) Well-known
allergens in dentures include residual monomers,
peroxides and metals. In patients with PMMA
hypersensitivity, the denture base should be
constructed from other polymeric materials to
which the patient is not allergic.(3)
Over the past 40 years, many denture-production
techniques other than pressing and/or molding
dentures from an MMA–PMMA system have been
developed, from light-cured paste systems to
microwave polymerization, and from the injection
molding technique to thermoplastic systems.
1) Assistant Professor, department of prosthodontics,
College of dentistry, University of baghdad.
2) Lecturer, department of prosthodontics, College of
dentistry, University of baghdad
One of the injection molding technique
advantages, is to allow directional control of the
polymerization process through the flask design,
and involves a constant flow of new
material from the sprue, thereby compensating for
polymerization shrinkage.(4) Thermoplastic
materials, such as polyamides (nylon plastics),
were introduced as early as the 1950s. With time,
several types of thermoplastic materials were
developed: acetal, polycarbonate, acrylic and
nylon (resin).(5,6)
The first thermoplastic nylons for dental
prostheses, Valplast (Valplast Int. Corp., Long
Island City, USA) and Flexiplast (Bredent,
Germany), were introduced in 1956.(7,8) Different
types of prostheses can be fabricated from these
materials via the injection-molding technique,(9)
which is used for the fabrication of flexible
denture-base prostheses.(10) Fluid resin is allowed
to flow into the mold cavity of these systems
through sprues created using sprue formers. The
resulting products are completely free of residual
monomers, accelerator systems and stabilizers.
Moreover, the thermoplastic products are always
homogenous in composition, and they respond
consistently during processing. Finally, these
thermoplastic materials have no adverse impact on
J Bagh College Dentistry Vol. 27(3), September 2015 The effect of different
Restorative Dentistry 2
the technician or the dental laboratory due to the
absence of hazardous materials.(11,12)
Thermoplastics are superpolyamides that belong to
the nylon family. Nylon can be defined as a resin
derived from dicarboxylic acid, diamine, amino
acid and lactams.(13) The injection-molding
technique is used for the fabrication of flexible
denture-base prostheses, which can be reversibly
liquefied upon heating. The molten material is
placed in a mold and allowed to cool down. A
range of thermoplastic materials are used in dental
mechanics.(14)
A new material called ThermoSens is superior to
standard polyamide materials. The flexibility of
this material can be controlled, and its shrinkage is
extremely low. Owing to its composition, a
homogenous color can be achieved, making this
material suitable for the preparation of full
dentures. The purpose of this study is to compare
vertical artificial teeth movement and surface
roughness between complete maxillary
ThermoSens dentures prepared by three different
investment materials: (a) dental stone, (b) silicone
putty (on the outer surface of the denture) and (3)
dental stone mixed with plaster of Paris (ratio, 1:1).
MATERILS AND METHODS
A complete maxillary denture with acrylic
teeth (Major, Italy) was made using a master cast
made from an ideal negative rubber mold
represented the upper edentulous arch by pouring
type III dental stone (Zhermack Elite Stone, Italy).
The record base was formed from +thermoplastic
acrylic cakes (BIOCRYL, Iserlohn, Germany) and
manipulated using a Biostar machine. The record
base had an even thickness of 2 mm. A horse
shoe–shaped block of wax was attached to the
record base to form the occlusion rim. The wax
was shaped such that the length from the highest
area of the labial flange (canine eminence) to the
occlusion edge was 22 mm and the length from the
highest area of the buccal flange (first molar
region) to the occlusal plane was 18 mm. The
width of the rim was approximately 4 mm
anteriorly and 7 mm posteriorly.(15) The maxillary
cast was mounted on the articulator by the aid of a
mounting plate, and maxillary anterior and
posterior acrylic teeth were arranged in a
monoplane occlusal scheme. The maxillary
denture was completely waxed. The sample was
removed from the articulator, and three wax
sprues were prepared. Two of these sprues were
attached to the posterior area of the maxillary
tuberosity, distal to the second molar, and the third
sprue was attached to the midline of the posterior
part of the palate (Fig. 1).
Figure 1: Three wax sprues are attached to the
posterior part of the maxillary denture.
Preparation of duplicated models:
Duplication of the simulation denture was
done by using a pourable silicone material
(VertexTM, Castasil 21) that is meant to be used for
the duplication of models supported by a plastic
container that provides rigidity for the duplicating
material (Fig. 2).
Figure 2: The denture is duplicated using a
pourable silicone duplicating material.
Thirty identical maxillary wax dentures were
prepared by placing the same size set of teeth into
the silicon mold, and then pouring the molten base
plate wax into the mold, which was leaved to cool
at 37° C for 2 hr before removal. This provided a
standard wax simulation of a maxillary denture
and thereby minimized variations in
polymerization due to the shape and size of the
dentures specimens.
Vertical reference point preparation
One metallic reference point placed in the canine
flange area of the dentures, on the facial surface of
the canine, in the first molar flange area and on the
buccal surface of the first molar (Fig. 3). These
reference points were used for vertical
J Bagh College Dentistry Vol. 27(3), September 2015 The effect of different
Restorative Dentistry 3
measurements. The first of all a tentative
measurements made by the aids of digital vernier
caliper to locate the position of the screws at the
wax stage before flasking, then a more precise
measurements were made by using micrometer
microscope to measure changes in the vertical.
Measurements before and after flasking
procedures.
On the facial surface of the canine and first molar,
a line was drawn with a soft marker perpendicular
to the occlusal plane and tangential to the distal
surface of the canine screw and mesial surface of
the molar screw by using a vernier caliper.
Another line was drawn perpendicular to the first
line, at a distance of nearly 16 mm from the canine
and 15 mm from the first molar. Then a screw pin
was attached to the intersection of the two lines in
the selected teeth (Fig. 3).
Figure 3: Reference point measurements.
Ten dentures were processed for each of the three
groups in the study:
Group I: conventional investment (dental stone)
Group II: silicone putty (VertexTM putty) used on
the outer surface of the denture
Group III: dental stone mixed with plaster of
Paris (ratio, 1:1)
Processing of the ThermoSens denture base
material and vertical measurements
After wax elimination from the denture, we used
an injection machine (VertexTM ThermoJect 22) to
inject ThermoSens capsules (Vertex ThermoSens,
capsule size XL) into a flask, which was then
allowed to cool to room temperature. Once cooled,
the flask was opened, and the denture was attached
to the base of a surveyor, so that the reference
points could be observed under a microscope. The
distance between the following reference points
was measured: (a) right canine–right screws, (b)
right molar–right screws, (c) left canine–left
screws and (d) left molar–left screws (Fig. 4).
Figure 4: Processed denture with the screws
attached at the canine and molar region
bilaterally
.Roughness test
For the surface roughness test, we prepared
30 specimens with dimensions of 65 mm × 10 mm
× 2.5 mm (length, width and thickness,
respectively) by using ThermoSens capsules
(Vertex ThermoSens, capsule size XL). Ten
samples were assigned for each of the three groups
mentioned in the dimensional changes test
(American National Standards Institute/American
Dental Association specification no. 12).
RESULTS
Dimensional changes
The means, standard deviations, standard error,
and minimum and maximum values of the
dimensional measurements in all groups are shown
in Table 1. One-way analysis of variance
(ANOVA) revealed significant differences (P <
0.05) in all measurements among all groups
(Tables 2 and 3). Both canine and molar region
measurements in group III significantly differed
from those in groups I and II (Tables 4 and 5). The
data for the right canine and molar regions, but not
the left regions, significantly differed between
groups I and II.
Roughness test
The mean, standard deviation, and minimum and
maximum values of surface roughness for groups
I, II and III are presented in Table 6. The mean
roughness value was higher in group I (dental stone
investment, 1.53 µm) than in group II (silicone
putty, 1.31 µm). The highest mean roughness value
was observed in group III (mixture of dental stone
and plaster of Paris, 1.61 µm).
One-way ANOVA table (Table 7) with the least
significant difference (LSD) test showed that
surface roughness was significantly influenced by
the material used in denture processing (P < 0.05).
Significant differences were found between groups
I and II (P < 0.012) and groups II and III (P <
0.001), but not between groups I and III (P <
0.362); (Table 8).
DISCUSSION
The dimensional characteristics of processed
denture bases are affected by many factors, such as
the type of acrylic, type of investment material,
J Bagh College Dentistry Vol. 27(3), September 2015 The effect of different
Restorative Dentistry 4
method of resin introduction and temperature used
to activate the polymerization process.(15)
In this in vitro study, all laboratory dentures were
measured in the wax stage and deflasking stage to
determine the effect of polymerization shrinkage of
the ThermoSens resin and the effects of investment
materials on denture dimensions.
Changes in teeth vertical measurements of
complete dentures
Group II showed less shrinkage in the dimensions
than did groups I and III. Table 1 show the effects
of different investment methods on the
polymerization of ThermoSens. The lowest mean
difference was observed in group II (silicone
putty), while the highest was observed in group III
(mixture of plaster and stone). These results may
be attributable to the type of investment material
and its effect on the amount of stress relaxation.
The harder the investment material, the more
difficult is the deflasking procedure, which results
in additional stress within the resin that is
subsequently released. This explains why the least
amount of shrinkage was detected in the group-II
samples. Silicone putty has a high tear resistance
(15) and can therefore be deflasked without
difficulty by using a scalpel to cut the putty and
liberate the sample. In comparison, groups I and
III, which involved gypsum investment, showed
more shrinkage, as the deflasking procedure was
the most difficult. These results are inconsistent
with those reported in some studies,(16–18) but agree
with those reported by Duke et al.(19) These
differences could be attributed to the use of
different denture base materials and measurement
methods in different studies.
In group II, the right and left vertical distances in
the molar region were similar. This finding could
be attributed to the less shrinkage of the resin, as
silicone showed better dimensional stability than
did dental stone, which expanded after processing.
In the canine region, the distances differed between
the right and left sides. This may be due to the
position of the cast and denture within the flask, as
stated by Wolfaardt et al.(20) Alternatively, it could
be attributable to the position in the injection
machine. The above results are inconsistent with
those reported by Abd,(16) who found greater
differences in the molar areas.
The differences in the vertical dimensions in the
canine and molar areas between the stages of
waxing and processing were highly significant on
both the right and left sides (Tables 2,3,4 and 5).
These results may be attributable to the amount of
polymerization shrinkage, thermal contraction of
the resin and mold, and the stress released during
deflasking. These findings agree with those of
Abd,(16) who reported that polyvinyl siloxane
duplicating materials produce better dimensional
stability, which is affected by the type of
investment method used.
Tables 3 and 5 show the differences in mean values
of the dimensional measurements between the
experimental groups. The differences in molar and
canine measurements between groups I and III, and
groups II and III were highly significant for both
the left and right sides. This could be attributable
to the effect of stress relaxation due to thermal
contraction and polymerization contraction of
putty silicone, which is mainly caused by the cross-
linking and rearrangement of bonds within and
between polymer chains.(21,22)
The molar and canine measurements on the right,
but not those on the left, significantly differed
between groups I and II. These findings may be
attributed to the investment material used, i.e., type
III dental stone, which has a setting expansion of
0.15%–0.25%. (21) Once the investment material is
set, the mold expands only slightly, as the effect of
the setting expansion of gypsum is reduced by
confining it within the flask.(23) The expansion of
the gypsum mold exceeds the polymerization
shrinkage of putty silicone, leading to the
expansion of the measured distances. The non-
significant differences could be attributed to the
erratic release of internal stress induced during the
deflasking procedure. The magnitude of
dimensional change depends on the conditions of
molding, the shape of the mold and the direction of
measurement.
Surface roughness
The ThermoSens denture base material is more
flexible than the commonly used PMMA.
However, the material polish-ability has not been
examined thoroughly. The surface roughness
(Table 6) was lowest (1.31 μm) in group II and
highest in group III (1.61 μm); the roughness in
group I was 1.53 μm. These results indicated that
the type of investment material affected the
roughness of the polishing surface of the dentures.
However, the roughness of the surface of the
ThermoSens material before polishing
significantly differ between groups I and II (Table
8), and this may be due to the use of diffrenet
investment materials (type III dental stone and
silicone putty). The non-significant difference
between groups I and III (Table 8) also may be
attributable to the use of different investment
materials (stone vs. mixture of dental plaster and
stone). The addition of plaster, which shows more
dimensional changes and a more porous surface,
affected the surface roughness. Our findings were
consistent with other studies(24–28) that have found
that the average roughness of unpolished
polyamide is 1.111 ± 0.178 µm. It is difficult to
J Bagh College Dentistry Vol. 27(3), September 2015 The effect of different
Restorative Dentistry 5
directly compare roughness values with other
studies because of differences in methodology
including; polishing methods, apparatus used for
measuring surface roughness and material types
used.
Groups II and III showed highly significant
differences in surface roughness (Table 8),
possibly due to physical properties differences of
the investment material used during processing and
the use of the injection procedure and overheating.
The injection molding temperature, pressure and
cooling rate must be standardized for optimal
denture-surface roughness
In conclusion, the use of silicone putty on the outer
surface of complete dentures before the investment
of the second layer will reduce the dimensional
changes and surface roughness of the ThermoSens
denture base material.
REFERENCES
1. Craig RG: Restorative dental material, (10 ed). St.
Louis, The CV Mosby Co., 1997, pp 127-136, 500-
540.
2. Kimono S, Kobayashi N, Kobayashi K, et al: Effect of
bench cooling on dimensional accuracy of heat-cured
acrylic denture base material. J Dent 2005;33:57-63
3. Fisher AA. Allergic sensitization of the skin and oral
mucosa to acrylic denture materials. J Am Med Assoc
1954; 156:238-42.
4. KiecSwierczynska M, Krecisz B. Allergic contact
dermatitis in a dental nurse induced by methacrylates.
Int J Occup Med Environ Health 2003;16:73-4.
5. Parvizi A, Lindquist T, Schneider R, et al. The
evaluation and advancement of dental thermoplastics.
Dental Town Magazine 2003;February:52-6.
6. Negrutiu M, Sinescu C, Romanu M, et al.
Thermoplastic resins for flexible framework
removable partial dentures.TMJ 2005; 55(3): 295-9.
7. Stern MN. Valplast flexible partial dentures.New
yourk state. Dent J 1964;30:123-36.
8. Negrutiu M, Sinescu C, Romanu M, et al.
Thermoplastic resin for flexible framework removable
partial dentures. TMJ 2005; 55:295-99.).
9. Keenan PL, Radford DR, Clark RK. Dimensional
change in complete dentures fabricated by injection
molding and microwave processing. J Prosthet Dent
2003;89(1):37-44.
10. Alvarez A, Cullivan B. Valplast – The flexible partial.
Dental Office 2003.
11. DiTolla M. Valplast flexible, esthetic partial dentures.
Chairside perspective magazine 2004;5(1):1-4.
12. Levin B and Richardson GD: Complete denture
prosthodontics. A manual for clinical procedures.
(17th) edition 2002, pp.54-55.
13. Carlsson GE and Magnusson T: Management of
temporomandibular disorders in the general dental
practice. Quintessence Publishing Co, Inc 1999. pp.
113.
14. Keenan PLT, Radford DR, and Clark RKF:
Dimensional change in complete denture fabricated by
injection molding and microwave processing. J
Prosthet Dent 2003;89(1):37-44.
15. Anusavice KJ. Phillip’s science of dental materials. 11th
edition. St.Louis: W.B. Saunders, 2008, pp. 145, 233,
234, 246, 247, 257, 258, 722, 737.
16. Abd S R. Tooth movement in maxillary complete
dentures fabricated with fluid resin polymer using
different investment materials.MSc.thesis in Prosthetic
department, University of Baghdad, College of
Dentistry,2012.
17. Grant AA and Atkinson HF: Comparison between
dimensional accuracy of denture produced with pour-
type resin and with heat-processed materials. J Prosthet
Dent 1971; 26 (3): 296-301.
18. Antonopoulos A N: dimensional and occlusal changes
in fluid resin dentures. J Prosthet Dent 1978; 39(6):605-
15.
19. Duke BS, Field H, Olson JW, et al. A laboratory study
of changes in vertical dimension using a compression
molding and a pour resin technique. J Prosthet Dent
1985;53(5):667-9.
20. Wolfaardt J, Cleaton-Jones P, Fatti P: The influence of
processing variables on dimensional changes of heat-
cured poly (methyl methacrylate). J Prosthet Dent
1986;55 (4):518-25.
21. Craig RG and Powers TM: Restorative dental materials.
11th edition. St. Louis: Mosby, 2002, pp. 341, 344, 346,
392-94, 397, 400-3, 636-9, 647,649, 656.
22. Bahannan S, Abd El-Hamid A, Abd Al-Halim M.:
Accuracy and reproducibility of reversible
hydrocolloids versus elastomers duplicating materials.
The Saudi Dent J 1995; 7(1):7-11.
23. Grant AA: Effect of the investment procedure on tooth
movement. J Prosthet Dent 1962; 12 (6):1053-8.
24. Wieckiewicz M, Opitz IV, Richter G, et al: Physical
Properties of Polyamide-12 versus PMMA Denture
Base Material, BioMed Research International Volume
2014, Article ID 150298, 8 pages.
25. Abuzar MA, Bellur S, Duong N, et al: Evaluating
surface roughness of a polyamide denture base material
in comparison with poly (methyl methacrylate) Journal
of Oral Science 2010; 52(4):577-81,
26. Kuhar M, Funduk N:Effects of polishing techniques on
the surface roughness of acrylic denture base resins. J
Prosthet Dent 2005; 93:76-85.
27. Oliveira LV, Mesquita MF, Henriques GEP, et al:
Effect of polishing technique and brushing on surface
roughness of acrylic resins. J Prosthodont
2008;17:308-311.
28. Berger JC, Driscoll CF, Romberg E, et al: Surface
roughness of denture base acrylic resins after
processing and after polishing. J Prosthodont
2006;15:180-186.
J Bagh College Dentistry Vol. 27(3), September 2015 The effect of different
Restorative Dentistry 6
الخالصة:
وخشونة نباألسنا التغيرات العمودية الحاصلة قياس بهدف الدراسة هذه أعدت. األسنان الكاملةأطقم تجهيز في تستخدم مادة الثرموسنس
كمادة السيليكون معجون استخدام مختلفة. ووجد إن غامسه مواد ثالث باستخدام األسنان أطقم سطح الثرموسنس المستخدم في تجهيز
تخفيض يمكن وبذلك. وقللت من خشونة األسطح األبعاد التغييرات خفضت الكاملة األسنان أطقم غامسه للثرموسنس المستخدم في تجهيز
.الطقم تجهيز خالل الثانية الغامسه الطبقة إضافة قبل األسنان على سيليكون المعجون طبقة إضافة خالل من السطح خشونة
Table 1: Descriptive statistics in all groups included in the vertical change in the measurements.
Table 2: One-way ANOVA of right and left canine measurements.
Table 3: LSD multiple comparisons test of dimensional changes in the canine region.
Groups N
Mean
(mm)
Std.
Deviation
Minimum Maximum
Molar
Measurements
Right
Group I 10 0.28 0.133 0.17 0.50
Group II 10 0.14 0.063 0.05 0.20
Group III 10 0.59 0.066 0.50 0.65
Molar
Measurements
Left
Group I 10 0.25 0.092 0.14 0.35
Group II 10 0.14 0.066 0.08 0.25
Group III 10 0.55 0.111 0.40 0.70
Canine
Measurements
Right
Group I 10 0.23 0.075 0.15 0.35
Group II 10 0.14 0.041 0.10 0.20
Group III 10 0.53 0.057 0.45 0.60
Canine
Measurements
Left
Group I 10 0.20 0.095 0.10 0.30
Group II 10 0.16 0.041 0.10 0.20
Group III 10 0.52 0.103 0.35 0.60
Sum of
Squares
Df Mean Square F Sig.
Right Side
Between Groups 0.834 2 0.417
130.919 HS Within Groups 0.086 27 0.003
Total 0.920 29
Sum of
Squares
Df Mean Square F Sig.
Left Side
Between Groups 0.764 2 0.382
59.64
HS
Within Groups 0.173 27 0.006
Total 0.937 29
Mean Difference Sig.
Right Side
Group I-Group II 0.090 S
Group I-Group III -0.300 HS
Group II-Group III -0.390 HS
Mean Difference Sig.
Left Side
Group I-Group II 0.048 NS
Group I-Group III -0.312 HS
Group II-Group III -0.360 HS
J Bagh College Dentistry Vol. 27(3), September 2015 The effect of different
Restorative Dentistry 7
Table 4: One-way ANOVA of right and left molar measurements.
Table 5: LSD multiple comparisons test of dimensional changes in the molar region.
Table 6: Mean surface roughness (µm), standard deviation, standard error of mean, and
minimum and maximum values in all groups.
Table 7: One-way ANOVA for surface roughness test
Table 8: LSD multiple comparisons test of surface roughness among all groups.
Mean Difference Sig.
Group I-group II 0.2224 S
Group I-group III -0.0785 NS
Group II-group III -0.3009 HS
Sum of
Squares
Df Mean Square F Sig.
Right Side
Between Groups 1.077 2 0.538 69.31
HS Within Groups 0.210 27 0.008
Total 1.286 29
Sum of
Squares
Df Mean Square F Sig.
Left Side
Between Groups 0.893 2 0.447 58.931
HS Within Groups 0.205 27 0.008
Total 1.098 29
Mean Difference Sig.
Right side
Group I-Group II 0.144 S
Group I-Group III -0.310 HS
Group II-Group III -0.454 HS
Mean Difference Sig.
Left side
Group I-Group II 0.116 NS
Group I-Group III -0.294 HS
Group II-Group III -0.410 HS
Group I Group II Group III
N 10 10 10
Mean (µm) 1.53 1.31 1.61
Std. Deviation 0.22 0.19 0.278
Minimum 1.04 1.02 1.005
Maximum 1.89 1.74 2.008
Sum of
Squares
Df Mean Square F Sig.
Between Groups 0.731 2 0.365
4.345
HS Within Groups 2.285 27 0.084
Total 3.016 29